There is known a percussive action machine (cf., U.S. Pat. No. 4,343,368, Int. Cl. B 25 D 9/04, published Oct. 8, 1982) for producing impact pulses to crush solid objects comprising a housing, a hammer with a tail piece, a power cylinder accommodating the tail piece of the hammer and filled with a compressed gas, a mechanism for reversing the movement of the hammer, and a means for decelerating the travel of the hammer as it executes an idle stroke including a cavity inside the housing opening at one side toward the interior of the power cylinder, and a cylindrical body having piston and annular projections and disposed inside said cavity for reciprocations therein. Part of the cavity confined between the annular and piston projections forms a hammer deceleration chamber, and is filled with a non-compressible liquid; inner surfaces of the housing defining the deceleration chamber have an annular flow restricting projection, whereas the surface of the cylindrical body between such projections has a special configuration providing continuity of the deceleration force acting on the hammer. The other part of said cavity confined by the piston projection of the cylindrical body is filled with the compressed gas and forms a return stroke chamber.
The return and work strokes of the hammer are accompanied by additional compression of the gas present in the power cylinder to store potential energy, whereas at the end of the return stroke the hammer is released from the return stroke mechanism, and under the action of the compressed gas exerted on the end face of the tail piece the hammer accelerates to execute a work stroke, after which the hammer again engages with the return stroke mechanism, and the hammer moves in the reverse direction. During an idle stroke, that is when the hammer at the end of the work stroke fails to meet a solid object to be crushed, or fails to expend all its energy to change the shape of this object, the hammer deceleration means is brought into action. Therewith, the hammer is caused to engage with the cylindrical body to move it forward, whereby the flow of liquid in the hammer deceleration chamber through a clearance between the annular projection and shaped surface of the cylindrical body is restricted, and the energy of the hammer is transformed to heat energy of the liquid to be dissipated.
The machine is provided with a hammer deceleration means, which effectively damps the residual energy of the hammer during an idle stroke thereof. However, after extensive use of the machine, elements of the hammer deceleration means tend to wear out to result in inadvertent collisions between the elements of the deceleration means and hammer, and subsequent failure of the machine.
There is further known a hydraulically operated percussive action machine (cf., West German Patent No. 2,223,292, Int. Cl. B 25 D 17/24, published June 8, 1978) having a hammer piston capable of reciprocations inside a housing of the machine to execute work and return strokes. The housing of the machine has a chamber in which the liquid under pressure acts on the hammer piston for the piston to execute a return stroke. The hammer piston is provided with an annular collar accommodated in said chamber, the front portion of this chamber acting as a damping or shock-absorbing means. The same pressure of liquid in the chamber acts on both end faces of the collar until the collar is outside the damping chamber. The machine also has a valve, which ensures a switchover of the liquid from feeding to discharge for the hammer to execute return and work strokes.
The work stroke is executed by the piston under the force of pressure of the working liquid on its rear end face, whereas the return stroke is executed by the pressure of liquid on the excess surface area of the hammer inside said chamber. When the hammer piston runs excessively over the length of its normal work stroke, the annular collar enters the damping chamber for the pressure of liquid to grow sharply therein, and by acting on the front end of the annular collar against the path of travel of the hammer piston tends to stop the latter. In this manner collision of the hammer piston with the housing of the apparatus is prevented.
The aforedescribed prior art machine operates reliably at low energy of impacts, for example, when used as a hand tool. Higher impact energy entails difficulties associated with displacing the working liquid from the chamber in the cource of the work stroke and deceleration of the hammer piston, when the hammer piston runs over the limits of the normal work stroke.